The present invention relates to improvements in spool type metering valves and more particularly to a spool type metering valve which is constructed such that the hydraulic losses associated with the entrance to the inlet port and the interior construction of the valve do not affect pressure readings extracted from the valve and therefore have no effect on the flow schedule produced by the valve.
Metering valves have been used throughout industry for a wide variety of purposes. Such valves generally include inlet and outlet ports for metering fluid flow therebetween. These valves have a variety of applications in which the rate of fluid flow is caused to vary as a function of a mechanical device. One such valve which is widely used includes a casing having a valve bore therein and includes axially spaced inlet and outlet ports. Disposed in the valve bore is a rotatable spool having circumferentially relieved areas which cooperate with the valve housing to define an axially restricted flow path between the inlet and outlet ports. The valve spool includes a land portion adjacent the outlet port having a cross-sectional area slightly greater than the cross-sectional area of the outlet port such that rotation of the valve spool and/or axial displacement of the spool within the bore varies the outlet area and thereby regulates the amount of fuel flowing from the outlet port. Such valves have a wide variety of applications and typically may be used to regulate fuel flow in a manner in which fuel flow varies linearly with spool position. For this purpose prior art valves of this type generally include transducers for measuring the pressure across the valve outlet port. These pressure readings are used to modulate a pressure regulating valve in the fluid medium supplying the metering valve such that the pressure difference across the metering valve outlet port is maintained constant thereby enabling the flow through the metering valve to be varied linearly with spool displacement. The relationship between fluid flow rate and the outlet port cross-sectional area may be represented mathematically as EQU Q = K A.sqroot..DELTA.P
where:
Q = fuel flow PA1 K = constant depending on valve geometery PA1 A = area of the outlet port PA1 66 P = pressure across the outlet port
By maintaining .DELTA.P constant, it can be seen from the above equation that the fuel flow Q will vary linearly as a function of the area A of the outlet port, which varies linearly as a function of spool displacement.
FIG. 1 illustrates a typical prior art valve of this type. The pressure across the outlet port 4 of the prior art metering valve 2 is generally extracted from the annuli 6 and 8 that surround the inlet and exit ports 3 and 4 respectively. Such prior art valves have demonstrated unsatisfactory performance particularly at relatively large fuel flow rates where such prior art valves generate excessive flow errors. These errors are a result of the hydraulic losses at the inlet and internal to the valve which depress the pressure between the inlet and outlet area thereby producing an error between the actual differential pressure across the outlet port and the sensed differential pressure. The hydraulic losses tend to disrupt the ability of the valve spool to produce a desired flow schedule versus spool displacement. This phenomena is particularly acute when the valve is used to meter large flow rates. As flow through the valve is increased, the internal losses become more significant and cause the flow schedule to drop significantly below the desired linear flow schedule. This phenomenon is illustrated in the graph of FIG. 3.